The production of phosphoric acid uses phosphoric rock, which is leached with sulfuric acid according to several well-known processes such as the dihydrate, hemi-hydrate hemi-dihydrate methods, or processes named for the companies that introduced the technology, for example Dorr-Oliver, Nissan H & C, Fissons, Lurgi, etc. The process produces the desired product, H3PO4, and a byproduct known as phospho gypsum or chemical gypsum, CaSO4.2H2O, according to the following reaction:Ca3(PO4)2+3H2SO4+6H2O→2H3PO4+3CaSO4.2H2O
This phosphor gypsum is basically made up of fine crystals, in tabular or rosette form, of sulfates of calcium dihydrate or hemi-dihydrate or mixtures thereof, depending on the process, with sizes of between 50 and 150 μm, as well as partially reacted rock or the insoluble fraction of phosphoric rock, namely silex, and oxides of iron, aluminum and magnesium, in addition to other compounds of the same metals but replaced by phosphates or polyphosphates. In addition, the phospho gypsum crystals also contain other inorganic compounds which have been absorbed or have replaced CaSO4 in the crystalline structure, including H3PO4, H2SO4 and HF, which are the products of incomplete reactions (H2SO4) and the usual shortcomings in solid-liquid separation; this gives the waste product an acidic character and causes one of the main problems concerning applications for construction materials or as a substitute for natural gypsum in the manufacture of cements.
During the production of H3PO4 between four (4) and five (5) metric tons of byproduct are produced per metric ton of acid, depending on the process used. This characteristic of the process requires the construction of large tailings dams or open areas for the retention and stockpiling of the damp solid. Given the acidic nature of the waste, there can be effects on the environment from leaching or dust generation, while certain phosphoric rocks also contain radioactive elements (uranium, thorium, radium) which require containment of the waste. Furthermore, disposal of the waste calls for large areas of land, which is accordingly rendered unusable for other purposes until closure of the plant and environmental recovery of the area. As an example, for the years 1981-1982, the annual worldwide production stood at 110 million metric tons.
The requirements for obtaining alternative methods of utilizing or disposing of phospho gypsum has driven a series of technological and research efforts aimed at providing an economical solution, or at least an environmentally acceptable one, to the phospho gypsum problem.
The technologies associated with the disposal of phospho gypsum are normally focused on evaluation of the environmental impact of the selected method, or the economies achieved through the application of the available technology.
Meanwhile, there is abundant research directed towards value-adding and re-utilizing phosphor gypsum in quite varied applications, and this gives a measure of the magnitude of the problem. Research into alternative applications has been oriented towards three principal areas: the agronomic area (fertilizers and soil conditioners), the production of construction material, and use as a raw material for obtaining chemical products and materials (sulfur, cements, calcium salts and other sulfates).
In the agronomic area, the principal use is in the conditioning of soils that are clay-like, acidic, or with a high content of sodium salts, and soils with erosion problems, low seepage, and as a source of sulfates for specific crops, or as low-formulation direct-application fertilizer.
The studies into obtaining other byproducts or raw materials were based on the application of technologies associated with pyrometallurgy, such as the decomposition of phospho gypsum in the presence of coke in order to obtain sulfur and cements, in addition to calcium sulfurates. Other studies were directed toward the preparation of fertilizers, K2SO4, through chemical digestion in the presence of nutrients such as KCI and NH4NO3 or NH3.
Finally, one area where research efforts are very abundant, and where there are proven industrial applications, is in materials for construction and civil works. We might expect such results in this area, given the large tonnages normally required to satisfy the demand for construction materials, especially where such materials are scarce or subject to economic competition.
In the international sphere, phospho gypsum has been utilized as an additive for regulating setting times, totally or partially replacing mineral gypsum. In the majority of cases, prior treatment of the phosphor gypsum has been necessary in order to reduce or eliminate the impurities which adversely affect the setting time, compression strength and other characteristics of prepared cement. Thus, we find that the first industrial-level application occurred in Japan, where phospho gypsum is heat treated by calcination at 800° C. (conversion to an anhydride, CaSO4) and mixed with bases such as CaO, then subsequently agglomerated to obtain a cement retardant. Likewise, the company ONODA developed a technology for obtaining gypsum for construction, stucco or plaster of Paris, but in this instance using chemical methods such as washing and neutralization, filtration and drying (U.S. Pat. No. 3,998,596, U.S. Pat. No. 3,928,053).
Similarly, Saltgitter patented a technology based on an upstream washing treatment in decanter tanks, neutralization with strong sodium or calcium bases, and hydro-cyclone separation, in order to eliminate impurities (free P2O5, F and others) and finally drying and calcination, in order to obtain a material that can be used in cement industry applications.
Other patents (CN1085879, CN1394823), operate using additives such as aluminum salts, clays, liquid byproducts of paper manufacture, CaO or Ca(OH)2 to treat the phospho gypsum until a material appropriate for use is achieved.
Other patents (CN1389421, CN1389422) refer to the application of unspecified modifying agents to treat chemical gypsum and subsequent mixing, molding, curing, and drying in a confined environment for up to 48 hours, and then adjustments or calcinations at between 600 and 900° C., and finally agglomeration (CN1394823).
The technological process associated with the present invention notably simplifies the number and type of distinct operations (elimination of upstream washing and filtering), in addition to reducing energy costs by operating at low temperatures (150° C. versus 800-1200° C. for other processes) during short periods (1-2 hours), reducing the quantities of neutralizing agents (10% maximum), as well as minimizing the volume of water required in the process; moreover the total processing time from damp phospho gypsum up to production of a pellet with adequate mechanical properties is less than 24 hours.
The product is presented in the form of pellets, which allows ease of use and reduction of fine [particles]
The final composition of the phospho gypsum obtained via this invention is similar to natural gypsums and compatible with Portland type cement formulations.